(Invited) Ultrawide Bandgap β-Ga₂O₃ Thin Films: Growths, Properties and Devices

Abstract

Ultrawide bandgap (UWBG) gallium oxide (Ga 2 O 3 ) represents an emerging semiconductor material with excellent chemical and thermal stability up to 1400 o C. It has a band gap of 4.5-4.9 eV, much higher than that of the GaN (3.4 eV) and 4H-SiC (3.2 eV). It exhibits high transparency in the deep ultraviolet (DUV) and visible wavelength region due to its very large bandgap. The monoclinic b-phase Ga 2 O 3 represents the thermodynamically stable crystal among the known five phases (α, β, γ, δ, ε). The breakdown field of β-Ga 2 O 3 is estimated to be 8 MV/cm, which is about three times larger than that of 4H-SiC and GaN. These unique properties make β-Ga 2 O 3 a promising candidate for high power electronic device and solar blind photodetector applications. More advantageously, single crystal β-Ga 2 O 3 substrates can be synthesized by scalable and low cost melting based growth techniques such as edge-defined film-fed growth (EFG), floating zone (FZ) and czochralski methods. For β-Ga 2 O 3 thin film synthesis, both molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) have been demonstrated to produce high quality and controllable doping films but with slow growth rates (2-10 nm/min). Halide vapor phase epitaxy (HVPE) using chloride precursors was demonstrated to grow β-Ga 2 O 3 films with fast growth rates (>5 μm/hr). Recently, we have developed a low pressure chemical vapor deposition (LPCVD) method to grow high quality β-Ga 2 O 3 thin films on both native Ga 2 O 3 and c-sapphire substrates with controllable doping and fast growth rates up to 10 μm/hr. In this talk, we present a study on the defects in β-Ga 2 O 3 thin films grown via LPCVD. The β-Ga 2 O 3 thin films were grown on native β-Ga 2 O 3 substrates and sapphire substrates using high purity gallium and oxygen as the precursors, and argon (Ar) as the carrier gas. The growth temperature ranged between 850 ˚C and 950 ˚C. The β-Ga 2 O 3 thin films were characterized by using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM images were taken with Helios 650. High resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) were taken using a FEI Tecnai F30 at 300 kV. Bright field, dark field, combined with two-beam condition TEM were used to characterize the defects in the as-grown films. From our studies, β-Ga 2 O 3 thin films grown on (010), (001) and (-201) β-Ga 2 O 3 substrates have shown different growth rates as well as different properties of interfacial defects. Improved LPCVD growths to suppress defects in β-Ga 2 O 3 thin films grown on different substrates will be discussed. Room temperature Hall measurements will be performed to understand the dependence of carrier transport on defects. In summary, the fundamental defect studies are performed on β-Ga 2 O 3 thin films grown via LPCVD on Ga 2 O 3 and sapphire substrates. The results from this study are important knowledge for power device applications.